Bootstrap switch circuit with over-voltage prevention

Abstract

In one embodiment, a bootstrap switch circuit has (i) a switch device that selectively provides a input signal as an output signal and bootstrap circuitry that provides a relatively high-voltage control signal to the gate of the switch device to turn on the switch device while preventing any over-voltage conditions from being applied to the switch device. The bootstrap circuitry includes a capacitor and a number of transistors configured as either switches or inverters. The circuit has two operating phases: one in which the capacitor gets charged while the switch device is turned off and the other in which the charged capacitor is isolated and used to generate the high-voltage control signal to be a fixed voltage difference above the current voltage level of the input signal applied to the switch device, thereby preventing an over-voltage condition.

What is claimed is: 
     
       1. Apparatus having a bootstrap switch circuit comprising:
 a switch device connected between an input node and an output node; 
 a first capacitor connected between the input node and a gate of the switch device; and 
 bootstrap circuitry selectively interconnecting a first plate of the first capacitor to the gate of the switch device and a second plate of the first capacitor to the input node, wherein: 
 during a first phase, the bootstrap circuitry is configured to (i) turn off the switch device and (ii) charge the first capacitor to a pre-charge voltage level which is no higher than a supply voltage level; 
 during a second phase, the bootstrap circuitry is configured to (i) cease charging the first capacitor and (ii) connect the first capacitor between the input node and the gate of the switch device, such that a voltage difference between the gate and a channel node of the switch device is maintained at or below a voltage tolerance level for the switch device; and 
 the bootstrap circuitry comprises:
 a first switch connected between a first supply voltage node and the first plate of the first capacitor, wherein a gate of the first switch is connected to the gate of the switch device; 
 a second switch connected between the second plate of the first capacitor and a second supply voltage node; 
 a third switch connected between the second plate of the first capacitor and the input node; 
 a first inverter connected between the first supply voltage node and the second plate of the first capacitor; and 
 a second inverter connected to an output signal of the first inverter and connected between the first and second plates of the first capacitor, wherein an output signal of the second inverter is connected to the gate of the switch device. 
 
 
     
     
       2. The invention of  claim 1 , wherein the bootstrap switch circuit has no more than two capacitors. 
     
     
       3. The invention of  claim 1 , wherein:
 during the first phase, (i) the second switch is closed, (ii) the third switch is open, (iii) the output signal of the first inverter is high, (iv) the output signal of the second inverter is low, and (v) the first switch is closed; and 
 during the second phase, (i) the second switch is open, (ii) the third switch is closed, (iii) the output signal of the first inverter is low, (iv) the output signal of the second inverter is high, and (v) the first switch is open. 
 
     
     
       4. The invention of  claim 1 , wherein the output signal of the second inverter is connected directly to the gate of the switch device. 
     
     
       5. The invention of  claim 1 , wherein the voltage difference between the gate and the channel node of the switch device is maintained at or below the supply voltage level. 
     
     
       6. The invention of  claim 1 , wherein the bootstrap circuitry further comprises:
 a fourth switch and a second capacitor connected in series between the first supply voltage node and the output signal of the second inverter; 
 a fifth switch and a sixth switch connected in series between the first supply voltage node and the output signal of the second inverter; and 
 a third inverter connected between the two plates of the second capacitor and having an output signal connected to the gate of the switch device. 
 
     
     
       7. The invention of  claim 6 , wherein the fourth switch is a thick-oxide device and the rest of the devices in the bootstrap circuitry are thin-oxide devices. 
     
     
       8. The invention of  claim 6 , wherein:
 during the first phase, (i) the second switch is closed, (ii) the third switch is open, (iii) the output signal of the first inverter is high, (iv) the output signal of the second inverter is low, (v) the first switch is closed, (vi) the fourth switch is closed, (v) the fifth switch is closed, (vi) the sixth switch is open, and (vii) the output signal of the third inverter is low; and 
 during the second phase, (i) the second switch is open, (ii) the third switch is closed, (iii) the output signal of the first inverter is low, (iv) the output signal of the second inverter is high, (v) the first switch is open, (vi) the fourth switch is open, (v) the fifth switch is open, (vi) the sixth switch is closed, and (vii) the output signal of the third inverter is high. 
 
     
     
       9. The invention of  claim 6 , wherein the voltage difference between the gate and the channel node of the switch device is maintained at or below two times the supply voltage level. 
     
     
       10. The invention of  claim 1 , wherein the bootstrap switch circuit has no more than one capacitor. 
     
     
       11. The invention of  claim 10 , wherein the bootstrap circuitry has no more than 7 switches. 
     
     
       12. The invention of  claim 1 , wherein the apparatus is an integrated circuit. 
     
     
       13. The invention of  claim 1 , wherein the supply voltage level is no higher than the voltage tolerance level of the switch device and any devices used to implement the bootstrap circuitry. 
     
     
       14. The invention of  claim 1 , wherein:
 the switch device is a thick-oxide switch device; and 
 the bootstrap circuitry comprises one or more thin-oxide devices having gate oxide thinner than gate oxide of the thick-oxide switch device. 
 
     
     
       15. Apparatus having a bootstrap switch circuit comprising:
 a thick-oxide switch device connected between an input node and an output node; 
 a first capacitor connected between the input node and a gate of the switch device; and 
 bootstrap circuitry selectively interconnecting a first plate of the first capacitor to the gate of the switch device and a second plate of the first capacitor to the input node, wherein: 
 during a first phase, the bootstrap circuitry is configured to (i) turn off the switch device and (ii) charge the first capacitor to a pre-charge voltage level which is no higher than a supply voltage level; 
 during a second phase, the bootstrap circuitry is configured to (i) cease charging the first capacitor and (ii) connect the first capacitor between the input node and the gate of the switch device, such that a voltage difference between the gate and a channel node of the switch device is maintained at or below a voltage tolerance level for the switch device; and 
 the bootstrap circuitry comprises one or more thin-oxide devices having gate oxide thinner than gate oxide of the thick-oxide switch device, wherein the bootstrap circuitry comprises:
 a first switch connected between a first supply voltage node and the first plate of the first capacitor, wherein a gate of the first switch is connected to the gate of the switch device; 
 a second switch connected between the second plate of the first capacitor and a second supply voltage node; 
 a third switch connected between the second plate of the first capacitor and the input node; 
 a first inverter connected between the first supply voltage node and the second plate of the first capacitor; 
 a second inverter connected to an output signal of the first inverter and connected between the first and second plates of the first capacitor, wherein an output signal of the second inverter is connected to the gate of the switch device; 
 a fourth switch and a second capacitor connected in series between the first supply voltage node and the output signal of the second inverter; 
 a fifth switch and a sixth switch connected in series between the first supply voltage node and the output signal of the second inverter; and 
 a third inverter connected between the two plates of the second capacitor and having an output signal connected to the gate of the switch device. 
 
 
     
     
       16. The invention of  claim 15 , wherein the supply voltage level is no higher than a voltage tolerance level for the thin-oxide devices. 
     
     
       17. The invention of  claim 15 , wherein the pre-charge voltage level is at most one-half of the voltage tolerance level for the thick-oxide switch device. 
     
     
       18. The invention of  claim 15 , wherein the voltage difference between the gate and the channel node of the switch device is maintained at or below two times the supply voltage level. 
     
     
       19. The invention of  claim 15 , wherein:
 during the first phase, (i) the second switch is closed, (ii) the third switch is open, (iii) the output signal of the first inverter is high, (iv) the output signal of the second inverter is low, (v) the first switch is closed, (vi) the fourth switch is closed, (v) the fifth switch is closed, (vi) the sixth switch is open, and (vii) the output signal of the third inverter is low; and 
 during the second phase, (i) the second switch is open, (ii) the third switch is closed, (iii) the output signal of the first inverter is low, (iv) the output signal of the second inverter is high, (v) the first switch is open, (vi) the fourth switch is open, (v) the fifth switch is open, (vi) the sixth switch is closed, and (vii) the output signal of the third inverter is high. 
 
     
     
       20. The invention of  claim 15 , wherein the fourth switch is a thick-oxide device and the rest of the devices in the bootstrap circuitry are thin-oxide devices. 
     
     
       21. The invention of  claim 15 , wherein the bootstrap switch circuit has no more than two capacitors. 
     
     
       22. The invention of  claim 15 , wherein the apparatus is an integrated circuit.